Various types of dental caps are known as a carrier or base on which porcelain veneer is applied, and therefore being a very important component of the finished dental crown. The crown retainer has predominantly been modelled in wax and cast in a fusing alloy.
In a new process according to European Patent Specification No. 104,320, round discs (FIG. 1) are cut out from a metal foil, prefabricated as star-shaped umbrellas (FIG. 2) and subsequently adapted manually to the tooth stump by means of special tools.
Furthermore, German Patent Specification No. 3,511,847 discloses a conical stove-pipe made of metal foil, in which several folded-out portions having a trapezoidal cross section are arranged over the periphery thereof; the stove-pipe, after being adapted to the tooth stump or model, has a three-layer outer surface at least in an edge region thereof, that is to say in the subsequent cervical region.
However, manual shaping is time-consuming, is subject to considerable fluctuations attributable to the individual performance thereof by the dental technician, and causes uneven folding leading to a large loss of mechanical strength. Random or angular folding results in stresses in the ceramic mass. In general, the production tolerance of the materials associated with manual shaping is very high because such must be manipulated by different dental technicians; material tests have shown that the laborious folding by hand of the umbrella or stove-pipe results in uneven inner faces which fail to form plane diffusion surfaces. The subsequent diffusion of the layer of pure gold by fusion to form a compact closed cap is often incomplete. The results are edge deformation, inaccuracy of fit and ceramic chipping.
According to German Offenlegungsschift No. 3,523,000, a metal foil which lays itself automatically against the tooth stump as soon as a force K is exerted on it from outside is used (FIG. 3). Under the effect of the above-mentioned force, the metal foil lays itself onto the surface of the tooth stump 1, linear vertical folds 2 being defined at equal distances from one another (FIG. 4). The number of loop-shaped folds and the interspaces are calculated so that, for example, a double folding strip 3 (FIG. 5) or a strip-like vertical strut 4 (FIG. 6) is obtained.
Accordingly, the tooth-crown retainer thereby formed consists of one or more metal layers, depending on the folding pattern and the arrangement in the pattern. Like the above-mentioned caps according to EP No. 104,320 and DE No. 3,511,847, at its lower edge or at the gingival margin, that is to say precisely where it is to provide a tight seal, it has a thickness which is irregular because of the folds, and this has a disruptive effect especially in the cervical and labial region and can lead to deformations in the subsequent dental crown and consequently to instability or only partial accuracy of the fit.